Blowing agent introduction into hopper of polymer foam processing

ABSTRACT

Methods and systems that include introducing blowing agent into a hopper of a polymeric foam system.

FIELD

The present invention relates generally to polymer foam processing and, more particularly, to methods and systems that include introducing blowing agent into a hopper of a polymeric foam processing system.

BACKGROUND

Polymeric foams include a plurality of voids, also called cells, in a polymer matrix. Polymeric foams are processed using a variety of techniques. For example, polymeric foams can be processed by injecting a physical blowing agent into the polymeric material within an extruder. For instance, many conventional systems inject blowing agent through a blowing agent port in the barrel of the extruder into a fluid stream of polymeric material within the extruder. The blowing agent may be mixed with the polymeric material to form a solution within the extruder. The solution may be, for example, injected into a mold to form an injection molded polymeric foam article. Such conventional systems may require modifications to standard extruder equipment (e.g., to extend length of the barrel to ensure sufficient mixing, to form a blowing agent port, etc.) and/or utilize relatively expensive equipment (e.g., blowing agent mass flow meter) to control the flow and introduction of blowing agent into the extruder.

Other prior art systems have attempted to simplify the design of polymer foam processing systems and reduce their costs. For example, certain systems have introduced physical blowing agent into a hopper of the extruder. Such systems generally have not controlled the amount of blowing agent introduced into the polymeric material. For certain foam processes, such an approach to blowing agent delivery may be adequate. However, in other processes that require relatively precise control over blowing agent delivery, such an approach may not be adequate.

Accordingly, there is a need for new blowing agent introduction techniques that may be used with polymer foam processes.

SUMMARY

Methods and systems including introducing blowing agent into a hopper of a polymeric foam system are described.

In one aspect, a system is provided. The system comprises an extruder including a screw configured to rotate in a barrel to convey a mixture of polymeric material and blowing agent in a downstream direction in a polymer processing space defined between the screw and the barrel. The system further comprises a mold connected to an outlet of the extruder. The screw is configured to periodically move in a downstream direction in the barrel to inject a shot of the mixture of polymeric material and blowing agent into the mold. The system further comprises a hopper configured to hold polymeric material pellets and blowing agent in a chamber volume. The hopper includes at least one inlet connectable to a source of the blowing agent. The hopper includes an outlet connectable to the polymer processing space in the extruder. The system further comprises a pressure regulator constructed and arranged to control the pressure of blowing agent supplied to the hopper. The system further comprises at least one processor and at least one storage medium having encoded thereon executable instructions that, when executed by the at least one processor, cause the at least one processor to carry out a method which comprises controlling a pressure of the blowing agent supplied to the hopper to a desired pressure using the pressure regulator based, at least in part, on the desired weight percentage of blowing agent in the shot of the mixture of polymeric material and blowing agent.

In one aspect, a method is provided. The method comprises providing a hopper configured to hold polymeric material pellets and supplying blowing agent to the hopper at a desired blowing agent pressure based, at least in part, on a desired weight percentage of blowing agent in the shot. The method further comprises supplying blowing agent and the polymeric material pellets to an inlet of an extruder including a screw configured to rotate in a barrel. The method further comprises conveying a mixture of polymeric material and the blowing agent in a downstream direction in the extruder and accumulating a shot of the mixture of polymeric material and blowing agent. The method further comprises injecting the shot into a mold to form a molded polymeric foam article.

In one aspect, a system is provided. The system includes an extruder including a screw configured to rotate in a barrel to convey a mixture of polymeric material and blowing agent in a downstream direction in a polymer processing space defined between the screw and the barrel. The system further includes a mold connected to an outlet of the extruder. The screw is configured to periodically move in a downstream direction in the barrel to inject a shot of the mixture of polymeric material and blowing agent into the mold. The system further comprises a blowing agent introduction system including a source of blowing agent and a pressure regulator. The system further comprises a hopper including a chamber volume having a port fluidly connected to the source of blowing agent. The hopper is configured to hold polymeric material pellets and blowing agent in a chamber volume. The hopper includes a first outlet configured to supply polymeric material pellets and blowing agent to the polymer processing space in the extruder. The system is configured to recycle blowing agent in the chamber volume to a location in the blowing agent introduction system upstream of the pressure regulator.

In one aspect, a system is provided. The system includes an extruder including a screw configured to rotate in a barrel to convey a mixture of polymeric material and blowing agent in a downstream direction in a polymer processing space defined between the screw and the barrel. The system further includes a mold connected to an outlet of the extruder. The screw is configured to periodically move in a downstream direction in the barrel to inject a shot of the mixture of polymeric material and blowing agent into the mold. The system further includes a blowing agent introduction system including a source of blowing agent and a hopper including at least a first chamber volume and a second chamber volume. At least one of the first or the second chamber volumes are configured to connect to the source of blowing agent and to hold polymeric material pellets and blowing agent.

Other aspects and features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a polymer foam processing system according to an embodiment.

FIG. 2 schematically illustrates a multi-chamber hopper assembly according to an embodiment.

FIG. 3 schematically illustrates a computing device suitable for use in connection with a polymer foam processing system according to an embodiment.

DETAILED DESCRIPTION

Methods and systems that include introducing blowing agent into a hopper of a polymeric foam system are described. The methods may utilize a control system that enhances control over the amount of physical blowing agent (e.g., nitrogen, carbon dioxide) introduced into the polymeric material being processed by the system. As described further below, the system controls the amount of blowing agent introduced, in part, by controlling the pressure of blowing agent supplied to the hopper to a desired amount. The desired pressure may be based, at least in part, on the desired weight percentage of blowing agent in the polymeric material being processed. As described further below, the methods may determine the desired blowing agent pressure from a variety of additional inputs which may relate to the blowing agent (e.g., blowing agent type), desired article characteristics (e.g., article weight), polymer characteristics (e.g., polymer type, polymer pellet bulk density) and equipment design (e.g., hopper chamber volume). The methods and systems are particularly well suited for processes that produce injection molded polymeric foam articles. In some embodiments, the methods and systems are designed to recycle blowing agent to reduce the amount of unused blowing agent. The methods and systems may also include a multi-chamber hopper design that facilitates re-filling the chamber(s) with polymeric material pellets, for example, when a chamber is empty.

Referring to FIG. 1 , a blowing agent introduction system 10 is used to deliver physical blowing agent to a polymer processing system 12. In this embodiment, system 12 is an injection molding system that includes an extruder 14 and a mold 16. A hopper 18 provides polymeric material (e.g., in the form of pellets) to the extruder. As described further below, the hopper includes a chamber volume in which pellets of the polymeric material are contained. According to the methods described herein, the blowing agent introduction system supplies blowing agent to the chamber volume in the hopper which can be supplied, along with polymeric material, to the extruder. As described further below, the blowing agent introduction system controls the pressure of the blowing agent supplied to the hopper based on the desired amount of blowing agent (i.e., blowing agent concentration) in the polymeric material and, in some cases, additional parameters.

The extruder includes a screw 20 designed to rotate within a barrel 22 to plasticate polymeric material. Heat (e.g., provided by heaters on the extruder barrel) and shear forces (e.g., provided by the rotating screw) act to melt the polymeric material to form a fluid polymeric stream which is mixed with the blowing agent supplied from the hopper. The mixture is conveyed in a downstream direction 24 in the extruder barrel by rotation of the screw. In some embodiments, the mixture is a single-phase solution with the physical blowing agent being dissolved in the polymeric material prior to injection into the mold. In the illustrated embodiment, a valve 29 is arranged between the outlet of the extruder and the inlet of the mold. A shot of the mixture (e.g., single-phase solution) may be accumulated downstream of the screw within the extruder causing the screw to retract in an upstream direction within the barrel. When suitable conditions have been reached (e.g., after a predetermined time period, at a predetermined screw position, etc.), the screw stops retracting and rotating to end the plastication period of the molding cycle. During the injection period of the molding cycle, the screw may be forced downstream within the barrel to inject the mixture into a cavity of the mold when valve 29 opens. The mixture is subjected to a pressure drop during injection which nucleates a large number of cells and a polymer foam article is formed in the mold. The screw may begin to rotate once again to begin another molding cycle. The method is typically repeated to produce multiple polymeric foam articles.

It should be understood that the polymer foam processing system may include a number of conventional components not illustrated in the figure. Though the blowing agent introduction system is illustrated as being used in conjunction with an injection molding system, it should be understood that the blowing agent introduction system may be used in conjunction with any other polymer processing system into which blowing agent is introduced including blow molding systems.

The blowing agent introduction system includes a blowing agent source 26 connectable to one or more port(s) 28 that are connectable to a chamber volume in the hopper. Conduit 36 is used to connect various components of the introduction system and to provide a pathway from the source to the blowing agent port(s). Upstream of the hopper, the blowing agent introduction system includes a pressure regulator 38 which, as described further herein, may be used to set the pressure of blowing agent supplied to the hopper at a desired level. In some embodiments, the blowing agent introduction system may include an accumulator 47 connected to an interchangeable bottle of blowing agent. In some embodiments, such as when a bottle does not supply blowing agent at a sufficiently high pressure, a pump may be connected to increase and/or maintain pressure of blowing agent in the introduction system. A control system 44 of the blowing agent introduction system may receive one or more inputs (e.g., relating to the desired amount of blowing agent introduced into the polymeric material which may be selected by an operator) and can provide output(s) to control the pressure regulator to supply a desired blowing agent pressure to the hopper. It should be understood that the blowing agent introduction system may include other standard components such as valves which may be used to selectively control blowing agent flow therepast. As described further below, the blowing agent introduction system may be configured to recycle residual blowing agent remaining in the hopper.

The control system may be any of the type known in the art such as a computing device, as described further below. As described above, the control system is capable of receiving input signals (e.g., from a user, from other components of the polymer processing system) and sending appropriate output signals (e.g., to components of the blowing agent introduction system such as the pressure regulator and/or the polymer processing system).

As noted above, techniques described herein involve supplying blowing agent to the hopper at a desired pressure. Such an approach may be used to supply a desired amount of blowing agent into the polymeric material (e.g., desired weight percentage of blowing agent material in the shot of polymeric material injected into the mold) which, for example, may be selected (e.g., as a value that is inputted to the system) by a user. As described further below, the desired pressure may be determined from a number of parameters in addition to the desired weight percentage of blowing agent in the polymeric material.

In some embodiments, the parameters may include characteristics relating to the equipment design. For example, the hopper chamber volume may be used as a parameter.

In some embodiments, the parameters may include characteristics relating to the polymeric material. For example, the type of polymer (e.g., resin type such as polypropylene, polyethylene, etc.), weight of polymeric material and/or polymeric material density may be used as parameter(s).

In some embodiments, the parameters may include characteristics relating to the injected molded article. For example, the weight (e.g., mass of polymeric material) of the injection molded article may be used.

In some embodiments, the parameters may include characteristics relating to the blowing agent. For example, the type of blowing agent (e.g., nitrogen, carbon dioxide) may be a parameter that is used in addition to the desired weight percentage of blowing agent in the polymeric material noted above.

In some embodiments, one aspect of determining the desired blowing agent pressure supplied to the hopper involves a step of determining the volume of blowing agent in the chamber in the hopper.

In some embodiments, one aspect of determining the desired blowing agent pressure supplied to the hopper involves a step of determining the amount of blowing agent that leaks out of the chamber volume of the hopper.

In some embodiments, one aspect of determining the desired blowing agent pressure supplied to the hopper involves a step of determining the maximum number of shots that may be achieved when using a hopper having a certain chamber volume.

The systems and methods described herein system may be used to introduce blowing agent into polymeric material within the extruder over a wide range of desired amounts. The desired blowing agent amount depends upon the particular process and is generally less than about 10% by weight of polymeric material and blowing agent. In many embodiments, the blowing agent level is less than about 5%, in others, less than about 3%, in others less than about 1%, in others less than about 0.5%, and still others less than about 0.1%, or even lower by weight of polymeric material and blowing agent mixture.

During an illustrative process, the source provides blowing agent to the introduction system. The source may supply any type of physical blowing agent known to those of ordinary skill in the art including nitrogen, carbon dioxide, hydrocarbons, chlorofluorocarbons, noble gases and the like or mixtures thereof. The blowing agent may be supplied in any flowable physical state such as a gas, a liquid, or a supercritical fluid. According to one preferred embodiment, the source provides nitrogen as a blowing agent. In another preferred embodiment, the source provides carbon dioxide as a blowing agent. In certain embodiments, solely carbon dioxide or nitrogen is used. Blowing agents that are in the supercritical fluid state after injection into the extruder, (optionally, before injection as well) and in particular supercritical carbon dioxide and supercritical nitrogen, are preferred in certain embodiments.

In some embodiments and as illustrated, the system is designed to recycle unused blowing agent. For example, the system may be configured to recycle residual blowing agent remaining in the chamber volume in the hopper after the polymeric material pellets in the hopper have been supplied to the extruder. In some cases, the residual blowing agent is removed from the hopper (e.g., via a port in the chamber volume) and re-circulated back into the blowing agent introduction system so that it may be used again. As shown in FIG. 1 , the blowing agent may be re-circulated via conduit 46. Conduit 46, for example, re-introduces the blowing agent into the blowing agent introduction system at a position upstream of the pressure regulator. In some embodiments, the blowing agent is re-introduced into an accumulator of the blowing agent introduction system. In some embodiments, re-circulated blowing agent may be re-introduced into the chamber volume of the hopper that contains unused polymeric material pellets.

In some embodiments, the polymer foam processing system includes a hopper having multiple chambers. For example, FIG. 2 schematically illustrates a multi-chamber hopper assembly 100 according to an embodiment. The multi-chamber hopper assembly, as shown, includes a first chamber 102 and a second chamber 104. The first and second chambers are connected to a source of polymeric material pellets (not shown). In some cases, and as shown, the hopper assembly includes a loader 106 which is configured to contain polymeric material pellets upstream of the hopper chambers. The assembly may include respective shut-off valves 108 arranged between the loader and inlets to the chambers which may be controlled to permit or prevent polymeric material pellets from entering the chambers. The assembly may also include blowing agent inlets 112 which are fluidly connected to the blowing agent source. Shut-off valves may be associated with inlets 112 to permit or prevent blowing agent from flowing into the chambers. Shut-off valves 114 may also be positioned at respective outlets 115, 117 of the chambers. In the illustrative embodiment, the outlets are connected to a third chamber 116. During use, one of outlet shut-off valves is open to permit supply of polymeric material pellets (and blowing agent) to the third chamber, while the other of the outlet shut-off valves is closed to prevent supply of polymeric material pellets and blowing agent to the third chamber. When all of the polymeric material pellets are used from one of the chambers the appropriate outlet valve is closed and the other outlet valve is opened to enable the other chamber to supply polymeric material pellets to the third chamber. The third chamber includes an outlet 118 that is connected to the polymer processing space so that polymeric material pellets and blowing agent may be supplied to the extruder. As illustrated, the third chamber also includes a blowing agent inlet that is fluidly connected to the blowing agent source.

Pressure may be maintained within the chambers that are supplying the polymeric material pellets and blowing agent to the extruder and reduced in the other chamber. That is, when the outlet of the first chamber is open and the outlet of the second chamber is closed, pressure may be maintained in the first chamber and the third chamber and may be reduced in the second chamber (e.g., to atmosphere, for example, to enable polymeric material pellets to be added to the second chamber). Similarly, when the outlet of the second chamber is open and the outlet of the first chamber is closed, pressure may be maintained in the second chamber and the third chamber and may be reduced in the first chamber (e.g., to atmosphere, for example, to enable polymeric material pellets to be added to the second chamber). When pressure is reduced in one or more chambers, it may be accomplished by releasing the blowing agent from the chamber. In some cases, the released blowing agent may be recycled as described above.

It should be understood that the multi-chamber hopper assembly may include additional components that are not illustrated. In some embodiments, blowing agent is not supplied to first chamber 102 or second chamber 104 and is only supplied to third chamber. In such embodiments, the first chamber and second chamber may not include blowing agent ports, while the third chamber may include a blowing agent port.

In general, the blowing agent introduction system may be used with the polymer processing system to produce any type of polymeric foam material. In some embodiments, the blowing agent introduction system may be used to introduce blowing agent into a polymer processing system that produces microcellular polymeric foam materials. In some embodiments, the microcellular polymeric foam materials produced may have an average cell size of less than 100 microns. It should be understood that polymeric foam materials having larger cell sizes may also be formed using the systems and methods described herein.

Techniques operating according to the principles described herein may be implemented in any suitable manner. Included in the discussion above are a series of flow charts showing the steps and acts of various processes that are described herein. The processing and decision blocks of the flow charts above represent steps and acts that may be included in algorithms that carry out these various processes. Algorithms derived from these processes may be implemented as software integrated with and directing the operation of one or more single- or multi-purpose processors, may be implemented as functionally-equivalent circuits such as a Digital Signal Processing (DSP) circuit or an Application-Specific Integrated Circuit (ASIC), or may be implemented in any other suitable manner. It should be appreciated that the flow charts included herein do not depict the syntax or operation of any particular circuit or of any particular programming language or type of programming language. Rather, the flow charts illustrate the functional information one skilled in the art may use to fabricate circuits or to implement computer software algorithms to perform the processing of a particular apparatus carrying out the types of techniques described herein. It should also be appreciated that, unless otherwise indicated herein, the particular sequence of steps and/or acts described in each flow chart is merely illustrative of the algorithms that may be implemented and can be varied in implementations and embodiments of the principles described herein.

Accordingly, in some embodiments, the techniques described herein may be embodied in computer-executable instructions implemented as software, including as application software, system software, firmware, middleware, embedded code, or any other suitable type of computer code. Such computer-executable instructions may be written using any of a number of suitable programming languages and/or programming or scripting tools, and also may be compiled as executable machine language code or intermediate code that is executed on a framework or virtual machine.

When techniques described herein are embodied as computer-executable instructions, these computer-executable instructions may be implemented in any suitable manner, including as a number of functional facilities, each providing one or more operations to complete execution of algorithms operating according to these techniques. A “functional facility,” however instantiated, is a structural component of a computer system that, when integrated with and executed by one or more computers, causes the one or more computers to perform a specific operational role. A functional facility may be a portion of or an entire software element. For example, a functional facility may be implemented as a function of a process, or as a discrete process, or as any other suitable unit of processing. If techniques described herein are implemented as multiple functional facilities, each functional facility may be implemented in its own way; all need not be implemented the same way. Additionally, these functional facilities may be executed in parallel and/or serially, as appropriate, and may pass information between one another using a shared memory on the computer(s) on which they are executing, using a message passing protocol, or in any other suitable way.

Generally, functional facilities include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically, the functionality of the functional facilities may be combined or distributed as desired in the systems in which they operate. In some implementations, one or more functional facilities carrying out techniques herein may together form a complete software package. These functional facilities may, in alternative embodiments, be adapted to interact with other, unrelated functional facilities and/or processes, to implement a software program application. In other implementations, the functional facilities may be adapted to interact with other functional facilities in such a way as form an operating system. In other words, in some implementations, the functional facilities may be implemented alternatively as a portion of or outside of an operating system.

Some exemplary functional facilities have been described herein for carrying out one or more tasks. It should be appreciated, though, that the functional facilities and division of tasks described is merely illustrative of the type of functional facilities that may implement the exemplary techniques described herein, and that embodiments are not limited to being implemented in any specific number, division, or type of functional facilities. In some implementations, all functionality may be implemented in a single functional facility. It should also be appreciated that, in some implementations, some of the functional facilities described herein may be implemented together with or separately from others (i.e., as a single unit or separate units), or some of these functional facilities may not be implemented.

Computer-executable instructions implementing the techniques described herein (when implemented as one or more functional facilities or in any other manner) may, in some embodiments, be encoded on one or more computer-readable media to provide functionality to the media. Computer-readable media include magnetic media such as a hard disk drive, optical media such as a Compact Disk (CD) or a Digital Versatile Disk (DVD), a persistent or non-persistent solid-state memory (e.g., Flash memory, Magnetic RAM, etc.), or any other suitable storage media. Such a computer-readable medium may be implemented in any suitable manner, including as computer-readable storage media 806 of FIG. 3 described below (i.e., as a portion of a computing device 800) or as a stand-alone, separate storage medium. As used herein, “computer-readable media” (also called “computer-readable storage media”) refers to tangible storage media. Tangible storage media are non-transitory and have at least one physical, structural component. In a “computer-readable medium,” as used herein, at least one physical, structural component has at least one physical property that may be altered in some way during a process of creating the medium with embedded information, a process of recording information thereon, or any other process of encoding the medium with information. For example, a magnetization state of a portion of a physical structure of a computer-readable medium may be altered during a recording process.

In some, but not all, implementations in which the techniques may be embodied as computer-executable instructions, these instructions may be executed on one or more suitable computing device(s) operating in any suitable computer system, including the exemplary computer system of FIG. 3 , or one or more computing devices (or one or more processors of one or more computing devices) may be programmed to execute the computer-executable instructions. A computing device or processor may be programmed to execute instructions when the instructions are stored in a manner accessible to the computing device or processor, such as in a data store (e.g., an on-chip cache or instruction register, a computer-readable storage medium accessible via a bus, a computer-readable storage medium accessible via one or more networks and accessible by the device/processor, etc.). Functional facilities comprising these computer-executable instructions may be integrated with and direct the operation of a single multi-purpose programmable digital computing device, a coordinated system of two or more multi-purpose computing device sharing processing power and jointly carrying out the techniques described herein, a single computing device or coordinated system of computing devices (co-located or geographically distributed) dedicated to executing the techniques described herein, one or more Field-Programmable Gate Arrays (FPGAs) for carrying out the techniques described herein, or any other suitable system.

FIG. 3 illustrates one exemplary implementation of a computing device in the form of a computing device 800 that may be used in a system implementing techniques described herein, although others are possible. It should be appreciated that FIG. 3 is intended neither to be a depiction of necessary components for a computing device to operate in accordance with the principles described herein, nor a comprehensive depiction.

Computing device 800 may comprise at least one processor 802, a network adapter 804, and computer-readable storage media 806. Computing device 800 may be, for example, a desktop or laptop personal computer, a personal digital assistant (PDA), a smart mobile phone, a server, a wireless access point or other networking element, or any other suitable computing device. Network adapter 804 may be any suitable hardware and/or software to enable the computing device 800 to communicate wired and/or wirelessly with any other suitable computing device over any suitable computing network. The computing network may include wireless access points, switches, routers, gateways, and/or other networking equipment as well as any suitable wired and/or wireless communication medium or media for exchanging data between two or more computers, including the Internet. Computer-readable media 806 may be adapted to store data to be processed and/or instructions to be executed by processor 802. Processor 802 enables processing of data and execution of instructions. The data and instructions may be stored on the computer-readable storage media 806.

The data and instructions stored on computer-readable storage media 806 may comprise computer-executable instructions implementing techniques which operate according to the principles described herein. In the example of FIG. 3 , computer-readable storage media 806 stores computer-executable instructions implementing various facilities and storing various information as described above. Computer-readable storage media 806 may store the various processes/facilities discussed above.

While not illustrated in FIG. 3 , a computing device may additionally have one or more components and peripherals, including input and output devices. These devices can be used, among other things, to present a user interface. Examples of output devices that can be used to provide a user interface include printers or display screens for visual presentation of output and speakers or other sound generating devices for audible presentation of output. Examples of input devices that can be used for a user interface include keyboards, and pointing devices, such as mice, touch pads, and digitizing tablets. As another example, a computing device may receive input information through speech recognition or in other audible format.

Embodiments have been described where the techniques are implemented in circuitry and/or computer-executable instructions. It should be appreciated that some embodiments may be in the form of a method, of which at least one example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

Various aspects of the embodiments described above may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any embodiment, implementation, process, feature, etc. described herein as exemplary should therefore be understood to be an illustrative example and should not be understood to be a preferred or advantageous example unless otherwise indicated.

Having thus described several aspects of at least one embodiment, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the principles described herein. Accordingly, the foregoing description and drawings are by way of example only. 

What is claimed:
 1. A method comprising: providing a hopper configured to hold polymeric material pellets; supplying blowing agent to the hopper at a desired blowing agent pressure based, at least in part, on a desired weight percentage of blowing agent in the shot; determining the desired blowing agent pressure, in part, from the volume of blowing agent in the hopper; supplying blowing agent and the polymeric material pellets to an inlet of an extruder including a screw configured to rotate in a barrel; conveying a mixture of polymeric material and the blowing agent in a downstream direction in the extruder; accumulating a shot of the mixture of polymeric material and blowing agent; and injecting the shot into a mold to form a molded polymeric foam article.
 2. The method of claim 1, wherein a pressure regulator is configured to provide the desired blowing agent pressure.
 3. The method of claim 1, wherein the source of blowing agent comprises nitrogen.
 4. The method of claim 1, wherein the mold comprises an injection mold.
 5. The method of claim 1, wherein the hopper includes multiple chambers.
 6. The method of claim 1, wherein the the mixture of polymeric material and blowing agent is a single-phase solution.
 7. The method of claim 1, wherein the molded polymeric foam article comprises a microcellular polymeric foam material having an average cell size of less than 100 micron.
 8. The method of claim 1, wherein the blowing agent is present in an amount of less than 1% by weight of the mixture of blowing agent and polymeric material. 